Measuring probes, for example, for determining pH values, ion concentrations, gas concentrations, conductivity, turbidity or other physical or chemical variables in a measured medium, have at least one end section immersed in the measured medium to take a measurement. Such measuring probes can be used, for example, in the area of industrial process measurements technology for determining and monitoring, the aforementioned measured variables of a process medium in a process. The end section immersed in the measured medium is also referred to as the immersion region of the measuring probe. In the immersion region, the measuring probe can have a measuring membrane, electrodes, coils, or one or a number of optical windows, for example, which serve to record and/or produce a signal correlated to the measured variable. In order to hold a measuring probe in a determined position in such a manner that the immersion region plunges into the measured medium with a predetermined immersion depth, for example, it is usually affixed to a securement apparatus, also referred to as a retractable assembly, in the selected position relative at a measuring point. There are various types of measuring points. A measuring point can be an open vessel in a laboratory, a vat in a clarification plant or a process vessel of a chemical plant open or closed to the environment, especially a pipe conveying media in such a plant, for example.
Typically, the aforementioned measuring probes have a probe shaft, which is frequently essentially rotationally symmetric. The term “rotationally symmetric” here refers to the geometric shape of the measuring probe or of the probe shaft. Such a measuring probe can be housed in a sleeve of the securement apparatus, for example. For affixing the measuring probe, the measuring probe can have an orbiting securement collar on the probe shaft; the securement collar is held shape interlocked by means of a releasably affixed coupling jacket in the sleeve; the releasably affixed coupling jacket presses the securement collar against a counterbearing, e.g. a ledge, provided in the sleeve. FIG. 1 shows such a measuring apparatus according to the state of the art. The measuring apparatus is described in further detailed below.
The measuring apparatus of FIG. 1 can be applied very advantageously at a measuring point under increased pressure, for example, for the execution of measurements in a measured medium, which is under increased pressure in a closed process vessel. The pressure reigning in the process vessel affects a force (arrow P) acting axially on the measuring probe in a direction facing away from the process. This force is opposed by the securement collar pressing against the ledge.
If the process pressure, however, is too high or an aging of the securement collar material arises, the tensile force acting on the securement collar can lead to a fracturing of the securement collar. This situation is shown in FIG. 2, which is explained in further detail below. If the securement collar fractures, the force due to the process pressure acting on the measuring probe affects a movement of the measuring probe in an axial direction away from the ledge, i.e. in a direction away from the process. In the case of a high pressure difference between the interior of the process container and the environment, the measuring probe can be completely forced out of the sleeve. Even if the measuring probe is only shifted within the sleeve, as shown in FIG. 2, the danger is that the immersion region of the measuring probe no longer plunges sufficiently deep into the process medium to permit sufficiently accurate measurements. Moreover, the axial shifting of the measuring probe within the sleeve can lead to a sufficient sealing of the process container relative to the securement apparatus or relative to the environment no longer being assured, so that medium can exit into the environment from the process container via the securement apparatus.